1. Field of the Invention
The present invention relates generally to surgical implants and, more particularly, to improved surgical implants formed of cobalt-chromium and its alloys, and a process of ion implanting the same.
2. The Prior Art
Surgical implants are widely used today in total joint replacements involving deteriorating or damaged hips, knees, shoulders, toes, fingers and elbows.
Surgical implants essentially comprise two parts: a metal part formed with an articulated surface designed to be received in and rub against a complementary load-bearing plastic surface of either an all-plastic part or a metal part with a plastic surface. The choice of metal for the metal part is either titanium and its alloys or cobalt-chromium and its alloys. The choice of plastic for the plastic part is, for the most part, ultra-high molecular weight polyethylene (UHMWPE). In use, both the metal part and its complementary plastic part experience abrasion and wear, but such abrasion and wear are more pronounced with respect to the plastic part. Until recently, the excessive wear of the plastic part has been tolerated and was considered acceptable eventually to wear away in a sacrificial manner. This was so since problems associated with the implant's attachment (or rather the lack of it) to bone to bone and with the rate of rejection of the implant by the body have been considered more significant than the wear of the plastic part. In fact to some extent, the wear representing the implant's useful life, has for the most part been designed into the implant. Recently, the problem of implant-attachment to the surrounding bone has been quite successfully addressed, inter alia, by applying porous coatings to the implant's surface. The problem of body-rejection has been countered with anti-rejection drugs and medication. These advances have caused the wear problem in the plastic component (UHMWPE) to be reassessed. Consequently, there is now a great deal of concern in the medical device industry focusing on the wear of the plastic component in an implant, since the wear has now become the determinant factor for the useful life of the implant in a patient.
In U.S. Pat. No. 4,743,493, entitled "Ion Implantation of Plastics" and assigned to said common assignee, Spire Corporation of Bedford, Mass., this wear problem was addressed with some success by ion implanting the plastic surface to a depth from about 0.1 to about 5 micrometers so as to increase its surface hardness and its resistance to chemical attack. A process for preventing surface discoloration of implants formed of titanium and its alloys, which titanium implants have been ion implanted to improve their wear performance, is disclosed in U.S. Pat. No. 4,693,760, entitled "Ion Implantation of Titanium Workpieces Without Surface Discoloration," also assigned to the said common assignee, Spire Corporation of Bedford, Mass. Implants made from cobalt-chromium and its alloys, while exhibiting good wear resistance, suffer from poor biocompatibility. A process of passivating the electro-chemically active surface of such cobalt-chromium alloys, hence inhibiting their corrosion, is disclosed in U.S. Pat. No. 4,743,308, entitled "Corrosion Inhibition of Metal Alloys," also assigned to the said common assignee, Spire Corporation of Bedford, Mass. This process essentially comprises the forming of a coating of biocompatible element from either platinum, gold or palladium on the surface of the cobalt-chromium implant, preferably by physical vapor deposition and exposing the thus coated surface to ion implantation.
The said copending application Ser. No. 07/514,503, filed Aug. 25, 1990, of which the present application is a continuation-in-part, has addressed the improving of the wear resistance of surgical implants made from titanium and its alloys by ion implantation. The present invention is intended, in contrast, to improve the wear performance of the plastic component of a surgical implant by ion implanting, not the plastic component but rather the metallic component of the implant, with the metallic part of the implant made, not from titanium and its alloys but rather from a cobalt-chromium alloy.